Magnetic disks and disk drives are conventionally employed for storing data in magnetizable form. Typically, one or more disks are rotated on a central axis in combination with data transducer heads positioned in close proximity to the recording surfaces of the disks and moved generally radially with respect thereto. Magnetic disks are usually housed in a magnetic disk unit in a stationary state with a magnetic head having a specific load elastically in contact with and pressed against the surface of the disk.
In operation, the magnetic disk is normally driven by the contact start stop (CSS) method, wherein the head begins to slide against the surface of the disk as the disk begins to rotate. Upon reaching a predetermined high rotational speed, the head floats in air at a predetermined distance from the surface of the disk due to dynamic pressure effects caused by the air flow generated between the sliding surface of the head and the disk. During reading and recording operations, the transducer head is maintained at a controlled distance from the recording surface, supported on an air bearing as the disk rotates. The magnetic head unit is arranged such that the head can be freely moved in the radial direction of the disk in this floating state allowing data to be recorded on and retrieved from the surface of the disk at a desired position.
Upon terminating operation of the disk drive, the rotational speed of the disk decreases and the head begins to slide against the surface of the disk again and eventually stops in contact with and pressing against the disk. Thus, the transducer head contacts the recording surface whenever the disk is stationary, accelerated from a stop and during deceleration just prior to completely stopping. Each time the head and disk assembly is driven, the sliding surface of the head repeats the cyclic operation consisting of stopping, sliding against the surface of the disk, floating in the air, sliding against the surface of the disk and stopping.
During reading and recording operations, it is desirable to maintain each transducer head as close to its associated recording surface as possible, i.e., to minimize the flying height of the head without contacting or damaging the data storage portion of the disk. This objective becomes particularly significant as the areal data recording density increases. Thus, a smooth recording surface is preferred, as well as a smooth opposing surface of the associated transducer head, for permitting the head and the disk to be positioned in close proximity, with an attendant predictability and consistency of behavior of the air bearing supporting the head. However, if the head surface and recording surface are too flat, the precision match of these surfaces gives rise to excessive stiction and friction during the start up and stopping phases, thereby causing wear to the head and recording surfaces eventually leading to what is referred to as "head crash." Thus, there are competing goals of reducing head/disk friction and minimizing transducer flying height.
In order to satisfy these competing objectives, the recording surfaces of magnetic disks are conventionally provided with a roughened surface to reduce the head/disk friction by techniques referred to as "texturing." Conventional texturing techniques involve polishing the surface of a disk substrate to provide a texture thereon prior to subsequent deposition of layers, such as an underlayer which is typically chromium or a chromium-alloy, a magnetic layer, a protective overcoat which typically comprises carbon, and a lubricant topcoat, wherein the textured surface on the substrate is intended to be substantially replicated on the outer surface of the magnetic disk.
The escalating requirements for high areal recording density impose increasingly greater requirements on thin film magnetic media in terms of coercivity, squareness, low medium noise and narrow track recording performance. In addition, increasingly high density and large-capacity magnetic disks require increasingly small flying heights, i.e., the distance by which the head floats above the surface of the disk in the CSS drive. The requirement to further reduce the flying height of the head imposed by increasingly higher recording density and capacity render it particularly difficult to satisfy the requirements for controlled texturing to avoid head crash.
Texture on magnetic recording media surfaces has been required, also, in data storage zones to orient the crystallization of the magnetic layer along circumferential lines to improve the signal-to-noise ratio and other magnetic performance. Conventional techniques comprise a mechanical operation, such as polishing. One such technique is to apply slurries with coolant for scratching the substrate surface. The slurries are inserted between a tape and the substrate with a certain normal force applied to the tape while the disk is in relative motion to the tape. The substrate surface is scratched by the slurry particles during this process, the resulting scratched lines known as surface texture lines. Because of the random distribution of the slurry particle sizes, these texture lines are randomly spaced with different scratch widths and depths. Also, because of the inconsistency of slurry concentration supplied to each disk, the scratch line width and depth vary from disk to disk. With conventional mechanical texturing techniques, it is extremely difficult to provide a clean textured surface due to debris formed by mechanical abrasions. Moreover, the surface inevitably becomes scratched during mechanical operations, which contributes to poor glide characteristics and higher defects. Such relatively crude mechanical polishing, with attendant non-uniformities and debris, does not provide a surface with an adequately specular finish or with adequate microtexturing to induce proper crystallographic orientation of a subsequently deposited magnetic layer on which to record and read information, i.e., a data zone.
FIG. 1 is illustrative of surface profiles obtained from typical mechanical texturing techniques. Asperities between scratch lines, which are created by the mechanical texturing method, vary greatly in size of up to the order of 50 .ANG. high on a surface of roughness average Ra of only about 5 .ANG.. The surface profile is a relatively random profile, with no specified number of peaks, nor defined heights of the asperities and depths of the valleys. As recording density requirements continue to increase, the size of each magnetic bit becomes smaller. As a result of random spacing of texture lines and random unacceptable scratch asperities and depths, more defects are found during magnetic testing.
An alternative technique to mechanical texturing comprises the use of a laser light beam focused on an upper surface of a nonmagnetic substrate. See, for example, Ranjan et al., U.S. Pat. No. 5,062,021, in which an NiP plated Al substrate is polished to a specular finish, and then the disk is rotated while directing pulsed laser energy over a limited portion of the radius, to provide a textured landing zone leaving the data zone specular. The landing zone comprises a plurality of individual laser spots characterized by a central depression surrounded by a substantially circular raised rim.
Another laser texturing technique is reported by Baumgart et al. "A New Laser Texturing Technique for High Performance Magnetic Disk Drives," IEEE Transactions on Magnetics, Vol. 31, No. 6, pp. 2946-2951, November 1995. See, also, U.S. Pat. Nos. 5,550,696 and 5,595,791.
The above-identified copending application Ser. No. 09/125,152, applies laser texturing to obtain an ultra-fine pattern with elongated asperities having low asperity height. While there are no deep valleys on the media surface, the elongated asperities are randomly elongated, created by a laser beam that is randomly modulated and focused on the data storage media surface. Although asperity elongation provides a more limited randomness in the circumferential direction, nonuniformity in height imposes negative effects on signal-to-noise ratio and magnetic performance as data density becomes increasingly greater.
Accordingly, there exists a need for a magnetic recording medium having data storage surfaces configured to accommodate the decrease in bit size concomitant with higher density storage. Such a configuration should provide an acceptable limit in the number of bits that are disqualified or missing in magnetic testing, which in the prior art are due to random spacing of deep scratches or texture lines. A further need exists for a laser micro-machining technique to form such high density storage surfaces in a practical manner.
Copending U.S. patent application Ser. No. 09/311,358, addresses these needs with a method for forming a textured data zone on a magnetic recording disk in which a focused laser beam is continuously applied to a substrate in a path of generally circumferential direction on the disk surface between inner and outer radii of a data storage zone. Application of the laser beam occurs while the substrate is rotated at a relatively constant first speed and the focused beam is moved radially at a relatively constant second speed, significantly slower. The resulting configuration is a continuous grooved structure in which a plurality of generally parallel and circumferential continuous ridges are separated by grooves. An underlayer, magnetic layer, protective overcoat and lubricant topcoat are then deposited, the textured surface of the substrate surface being substantially replicated on subsequently deposited layers. One or more data storage tracks are then formed in the magnetic layer of each of the resulting ridges.
This method provides improved magnetic layer crystallization orientation with a more regular geometric texture configuration within acceptable ranges. However, use of a single laser beam, accurately focused at a very fine point on the substrate surface, requires continuous application through a number of revolutions equal in number to the number of revolutions of the groove that is formed. As the substrate surface is traversed radially at rate in the range of about 0.001 inches per second (IPS) to 0.010 IPS, the time during which relatively constant power and high accuracy focus must be continuously administered between the inner and outer disk radius is considerable.
There continues to be a need for new laser-optical techniques for micro-machining on magnetic recording media substrate surfaces to form regularly-spaced ridges/grooves with enhanced magnetic layer crystallization orientation.